Method of producing electrical capacitors with semiconductor layer



METHOD OF PRODUCING ELECTRICAL CAPACITORS WITH SEMICONDUCTOR LAYER FiledNov; 9, 1967 Fig.7

Fig.2

Nb 2 5-x n Z AIR INVENTOR ERIC If. RAH/(N555 A ORNEY United StatesPatent US. Cl. 29-25.41 5 Claims ABSTRACT OF THE DISCLOSURE The anode oftantalum or niobium coated with a dielectric oxide layer on asemiconductive layer of manganese dioxide is subject to an anodictreatment in molten metal at a temperature higher than 150 C. with a DC-voltage not higher than the nominal DC-voltage of the capacitor.

BACKGROUND OF THE INVENTION The invention relates to a method to produceelectrical capacitors in which on a body of valve-type metal at first adielectric oxide layer is produced whereupon a semiconductor layer isapplied onto said oxide layer, said semiconductor layer being coveredwith a conductive layer.

Such capacitors are known in the literature as solid electrolyticcapacitors, although, in contrast to the electrolytic capacitors, noeletrolytic current flow takes place.

The body of valve-type metal is understood in the present invention asany arbitrary form, e.g. solid bodies, sintered bodies, metal sheets,wires, etc,

The valve-type metal in such capacitors is frequently tantalum orniobium, although other metals, too, may be used as e.g. aluminum,titanium, zirconium, hafnium, etc.

When producing such a capacitor at first a dielectric oxide layer isproduced onto the valve-type metal by an anodic oxidation and on saidoxide layer a semiconductor layer is applied.

This semiconductor layer may be manganese dioxide, produced by a thermaldecomposition of a dilution of manganese nitrate. But also othersemiconductors for said layer may be used, as e.g. lead dioxide producedfrom lead compounds known to the art.

In order to provide a feeder lead to the semiconductor layer aconductive layer is applied onto said semiconductor layer. It isprincipally possible to apply a metal layer of suitable metal or of analloy by spraying, evaporation or any other suitable manner. Because,however, these metal layers poorly adhere to the semiconductor layer atfirst a graphite layer is applied to said semiconductor layer upon whichgraphite layer the metallic layer is applied thereupon.

It has been found that the electrical properties of such capacitors areunstable, in that in the course of the time the electrical properties ofthe capacitor change.

This impairment of the electrical properties has different reasons andconsequently, various measures have been proposed for improvement, whichmeasures however did not lead to results desired.

For example, it has been suggested that the semiconductive oxide layerbe artificially aged at a temperature of 300 C.

It is also known to remove, prior to the application of thesemiconductor layer, the impurities from the oxide layer at atemperature of between 1700 and 2600 C.

The periodical Journal of Electrochemical Society, 1963, pages 12641271also describes a heat treatment of tantalum oxide layers at temperaturesabove 200 C. It

is stated in said article that the capacity is thereby increased, but atthe same time the electrical properties of the capacitor becomes moredependent on the voltage.

SUMMARY OF THE INVENTION The present invention proposes a method toproduce electrical capacitors with a semiconductor layer with the aid ofwhich the electrical properties are stabilized, thereby maintaining theadvantageous electrical properties of such capacitors. The methodmoreover shows the advantage that it can be performed together with thenecessary production process of applying a conductive layer, so that noadditional process is necessary to manufacture the capacitor.

The method according to the invention is characterized in this that thevalve-type metal body provided with the dielectric oxide layer and thesemiconductor layer, has to undergo an anodic treatment in molten-metalat a temperature of at least 150 C.

With the aid of this method the electrical properties of thecapacitorare stabilized together with the application of a conductive layer, butthe disadvantages caused by a pure heat treatment are avoided by theadditional current treatment.

Although it is already known to apply a voltage during the decompositionof a manganese compound to produce a semiconductor layer of manganesedioxide, the advantages of the method according to the present inventionare thereby not obtained, because the heat and the current treatment isapplied only after the complete application of the semiconductor layer.

BRIEF DESCRIPTION OF THE DRAWINGS The above-mentioned and other featuresand objects of this invention will become more apparent by reference tothe following description taken in conjunction with the accompanyingdrawings, in which:

FIGURE 1 is a schematic diagram of the apparatus for producing theimproved capacitor of this invention; and

FIGURE 2 is a schematic representation of the system Nb-Nb O -MI1ODESCRIPTION OF THE INVENTION Solid electrolyte niobium capacitors can bemade in a similar way to solid electrolyte tantalum capacitors, Theniobium metal is anodized in an electrolyte, which causes the formationof the dielectric, Nb O on the surface of the metal. Manganese dioxideis then brought into contact with the Nb O by the thermal decompositionof manganese nitrate, Mn(NO The cathode connection is brought about bythe application of a layer of graphite followed by a layer of conductivesilver and finally a layer of solder. The capacitors can then be eitherembedded in an epoxy resin or soldered in a hermetically sealed housing.Capacitors made by the above method when aged at C. with working voltageapplied, show the unwanted effect that the capacitance and dissipationfactor values fall.

It was found that the capacitance of the finished capacitor was alwayshigher than that of the formed anode and capacitance of the agedcapacitor was always slightly lower than that of the formed anode. Therise and fall of capacitance varied between 10% and 50% depending uponthe method of production used. In other words, it can be said that theforming process produces a capacitor with a certain capacitance. Thiscapacitance rises during the subsequent manufacturing steps. Finallythis rise of capacitance is removed by the aging process,

However, this aging process is a costly step to be included in theproduction of the capacitor, therefore a simpler method of obtaining thefall in capacitance was sought, The novel method of this invention wasto apply a DC voltage across the capacitor during the tinning operation.As the temperature of the solder bath is about 180" C., much higher thanthe normal aging temperature of 85 C., the method could be regarded asan accelerated aging process.

This was verified in an experiment. The capacitors used in the tinningexperiments were from the same batch of 25 volt capacitors with anaverage formed capacitance of 9/Lf. The capacitors were all measuredbefore and after a normal tinning at 185 C. As shown in FIGURE 1, thecapacitor 1 was then subjected to a further tinning operation, this timewith an applied DC. voltage from power supply 2 between the capacitor 1and the tin-bath 3 which was kept in a molten state by the heatingelement 4. The temperature, voltage and time were varied in order todetermine their relationship with the change of capacitance.

The system Nb-Nb O -graphite and the NbNb O electrolyte together withthe influence of heat-treatment upon them has been previously reported.The similarities between the niobium systems compared with those of thetantalum systems have been discussed by D. M. Smyth and T. B. Tripp ofthe Electrochemical Society 113- (1966) 1048. The basic model describedtherein is taken as the foundation of the theory which is to be putforward here to explain the behavior of the Nb-Nb O -MnO structure.

It is well known that niobium oxidizes in air at temperatures above 150C. whereas tantalum only begins to oxidize at temperatures above 260 C.It has been reported that at a given temperature the presence of ananodic film on the metal decreases the gross oxygen consumption rate,which is then dependent on the film thickness.

It is also a fact, that manganese dioxide acts as a cartier of oxygen.For example, manganese dioxide is much used for the hardening of oils insiccatives, its transporting property being used to activate oxygen fromthe atmosphere and carrying it to the unsaturated fatty acids therebyaccelerating the hardening process. Using these facts together with theproposed model in FIG. 2 we can explain the changing capacitance anddissipation factor as follows:

After the formation of the oxide, it is assumed that the layer istotally composed of niobium pentoxide, Nb O the complete layer acting asdielectric. During the decomposition of the manganese nitrate tomanganese dioxide the whole system is heated up to temperatures between150" C. and 300 C. Because of the affinity of niobium for oxygen at thistemperature oxygen ions will be drawn from the region next to the metal.The diffusion through the Nb O of oxygen ions from the manganese:lioxide wili attempt to replace these deficiencies. However, thisdiffusion rate is not so great as to completely replace the vacanciesand an equilibrium condition will 3e set up in which an intermediatelayer of oxide with )xygen vacancies exists next to the niobium metal.When :he system is returned to room temperature, the flow of )xygen ionsstops and the intermediate layer is frozen in ;itu. In this oxygenvacancy region the vacancies can be regarded as donor centers. Thereforethis region will :xhibit n-type semiconductivity. Because of this thethickiess of the effective dielectric will be reduced, giving rise :0 anincrease in capacitance and dissipation factor (the aemi-conductingregion acting as an extra resistance in ieries with the capacitance).

' If a DC. voltage is now applied to this system at a high temperaturethen the following can take place:

(1) The oxygen ions near the niobium-niobium oxide nterfaces will bedrawn into the metal as above.

(2) Oxygen ions supplied by the MnO will be ac- :elerated by the appliedvoltage field through the oxide llld fill up to vacancies left by action1.

The excess of oxygen ions supplied by action 2 does lot affect the rateof reaction 1 which takes place accordng to Arrhenius Law.

If the temperature and the voltage can be adjusted so that the diffusionrate of action 2 is higher than that of action 1, then the result willbe that the region of oxygen vacancies will be reduced. Upon cooling andfreezing in of this condition, the capacitance and dissipation factorwill have fallen towards their original values. Furthermore, if the timeof the application of voltage at temperature is sufiiciently long enoughfor equilibrium to be reached then the capacitance and dissipationfactor will fall to their original values.

The first result considered was the connection between the fall incapacitance during life-test and that which occurs when the capacitanceis measured with an applied DC. bias voltage. The fall in capacitancedue to the bias voltage before the life-test indicates the existence ofan intermediate layer. The voltage field draws the conduction electrons(at room temperature electrons are the predominant current carrier) inthe intermediate zone towards the metal increasing the effective widthof the dielectric thereby decreasing the capacitance and dissipationfactor. After hours life-test the capacitance has fallen and now theapplication of a bias voltage has very little effect indicating that theintermediate layer has now practically disappeared.

A typical behavior of the capacitors when tinned with and withoutvoltage is given in table 1 below:

It was found that the optimum conditions necessary to obtain practicallythe same fall in capacitance which takes place during the life-test area temperature of C., an applied voltage of 15 v. (20 v. being too nearthe breakdown point) and a time of 5-6 minutes.

It was also found that when the temperature rises from to 230 C. thediffusion rate of the ions from the oxide to the metal increasesfourteen fold. This amount of increase is to be expected as the generalformula for the temperature dependence of diffusivity is of the type D=Dexp (E/kT The explanation for the fact that the capacitance did notincrease during this tinning operation, is as follows: the capacitancewill have risen during the manganizing. However, the reforming iscarried out at room temperature with a reforming voltage only half thatof the forming voltage.

The results of the experiments carried out have supported the proposedtheoretical explanation of the effect which takes place when a niobiumcapacitor is operated at high temperatures with an applied DC. voltage.The method of applying a DC. voltage during the tinning operation in theproduction of niobium capacitors is now in use. Preliminary experimentswith tantalum and aluminum solid electrolyte capacitors have shownsimilar effects; the effect with tantalum, however, is not sopronounced.

With the method according to the invention the semiconductor layer canbe applied of course in several partial layers, as known, eventuallywith a carried out improvement of the anodic oxide layer in betweenthose partial applications.

All metals and alloys may be used as metal melt which are suitable toproduce a metal layer for the capacitor as a conductor lead, the meltingpoint of which is not higher than the temperature provided for thetreatment. The melting point of the metal or of the alloy used willnormally be beiow 150 C.

It is known to produce the metallic conductive feeder layer incapacitors with semiconductor layers of leadtin alloys (US. Patent3,100,329) or of metals such as copper, silver or nickel.

In the method according to the invention the capacitor, provided with adielectric oxide layer and a semiconductor layer is dipped into a metalmelt and a voltage is applied to the valve-type metal body and to themelted metal which approximatley corresponds to the nominal DC- voltageof the capacitor.

Thereby the capacitor plays the role of the anode.

It revealed that, when using niobium as value-type metal, the inventivemethod can be advantageously performed at a temperature of between 170and 190 C.

For an improved adhesion of a metal layer, formed by the material of themelt, it is advantageous at first, to apply a graphite layer onto thesemiconductor layer and to apply to the graphite layer a layer ofconductive silver, before the anodic treatment in the metal melt iscarried out.

Normally the advantages of the method according to the invention areobtained already during a rather short treatment. In many cases it issufiicient to carry out the treatment for five minutes.

When removing the capacitor body out of the metal melt a layer of thematerial remains on the body of the capacitor, forming the feeder leadto the semiconductor layer. A feeder or terminal wire can then besoldered onto said layer.

During the treatment according to the invention not only are theelectrical properties stabilized, but they are partly improved. Forexample, the capacity and the residual current is brought to improvedvalues without making these values more dependable on the voltage.

The method according to the invention can be applied for all capacitorswith a semiconductor layer, but is particularly suitable for capacitorshaving tantalum and niobium as valve-type metal.

While I have described above the principles of my invention inconnection with specific apparatus, it is to be clearly understood thatthis description is made only by way of example and not as a limitationto the scope of my invention as set forth in the objects thereof and inthe accompanying claims.

I claim:

1. The method of producing electrical capacitors wherein a body ofvalve-type metal from the group consisting of tantalum, niobium,aluminum, titanium, zirconium and hafnium is provided with a dielectricoxide layer, onto which oxide layer a semiconductor layer is appliedwhich is covered by a conductive layer, the improvement comprisingdipping the valve-type metal body, covered with the dielectric oxidelayer and with the semiconductor layer into a metal melt heated to atemperature of at least C., applying a voltage to said metal body andsaid melt and withdrawing said body after a time sutficient to stabilizethe electrical characteristics of said body.

2. Method according to claim 1, wherein the treatment is performed withthe nominal DC-voltage of the capacnor.

3. The method according to claim 1, wherein niobium is the valve-typemetal and the temperature is maintained between and C.

4. The method according to claim 1 comprising the further steps ofapplying a graphite layer onto the semiconductor layer, and thenapplying a conductive silver layer onto said graphite layer beforedipping the metal body into the metal melt.

5. The method according to claim 1, wherein a layer of the material ofthe melt remains on the capacitor body as a metallic lead.

References Cited UNITED STATES PATENTS 2,731,706 1/ 1956 Grouse 2925.423,073,943 1/ 1963 Girling 2925.41 X 3,279,030 10/1966 Wagner 2925.42 X3,284,684 11/1966 Gaenge 2925.42 X 3,292,053 12/1966 Giacomo 29-2541 X3,305,914 2/1967 Raue 2925.42 3,337,429 8/1967 Zind 29570 X 3,345,54310/1967 Sato 29570 X FOREIGN PATENTS 742,379 12/ 1955 Great Britain.

WAYNE A. MORSE, JR., Primary Examiner US. Cl. X.R. 29-570

